-LRB- CNN -RRB- -- How did Shannon Reynolds make it through several layers of security unquestioned and end up on the tarmac at Hartsfield-Jackson Atlanta International Airport ?

That 's what Delta Air Lines , the Transportation Security Administration and the Atlanta Police Department are trying to figure out .

Around 5 a.m. on February 23 , Reynolds was simply trying to park at an off-site parking lot in time to catch a flight on Spirit Airlines , she told CNN .

She did n't notice she was pulling into a Delta employee parking lot , where she said she was waved through two security checkpoints and allowed to park . Wearing a sundress and carrying her suitcase , she got on a Delta employee shuttle without being asked for identification . None of the employees on board was wearing a uniform , she said , so she said she did n't realize it was for employees only .

`` When I got to the airport , I looked around and I realized this is not where I was supposed to be . It was the tarmac with a big airplane sitting there , '' Reynolds said .

Reynolds alerted the shuttle driver , but she was still allowed to get out of the bus onto the tarmac . A ramp agent used his badge to take her up the stairs into the airport passenger area .

`` Here I was sitting in the middle of the C Terminal without having to go through security or do anything , '' Reynolds said . The airport employee who let her into the secure area told her to go downstairs and go through security as she normally would , Reynolds said . Inside the terminal someone finally stopped her and asked for her identification .

That started the investigation into how she had made it so far without being checked . -LRB- In a police report on the incident , the ramp agent she followed into the concourse said he assumed she was an employee because of all the checkpoints she had to pass . -RRB-

`` Airports are responsible for implementing security programs that restrict access to secure areas , '' said TSA spokesman David Castelveter in a statement . `` TSA is conducting an investigation to determine how an individual without proper credentials accessed the secure area of Hartsfield-Jackson Atlanta International Airport via an airline employee parking lot . ''

Delta is re-emphasizing the checking of employee identification at Delta employee parking lots , according to the airline .

`` We have taken aggressive action with our security vendors at the parking lot to emphasize that all security procedures , especially verification of credentials , are strictly adhered to , '' said Delta spokesman Morgan Durrant in a statement .

The police report states that Reynolds `` was transported back to her vehicle and allowed to move her car '' from the parking lot , noting that the delay caused her to miss her 6:30 a.m. flight to Fort Lauderdale , Florida , then on to Costa Rica .

Reynolds told CNN she was able to catch a later flight to Costa Rica , from where she recently returned .

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A traveler trying to make an early-morning flight accidentally parked in a Delta employee lot

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She says she was allowed to get on an employee bus without being asked for identification

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Transportation Security Administration is investigating how incident happened